The present invention relates to a free abrasive slurry composition containing free abrasive particles which is advantageously used in uniform grinding or processing of a composite material composed of a plurality of materials having different hardness from each other, without causing difference in the grinding amount between soft materials and rigid materials, i.e. selective grinding, during lapping and polishing processes of the composite material.
Recently, higher performance and higher function have been more and more demanded for optical parts, electronic parts, precision machine parts or the like, and wide range of materials have been used for such parts, such as metallic crystalline materials, ceramics, glass, plastics and so forth.
As one of the manufacturing processes of such parts, grinding or polishing of a composite material composed of a plurality of materials having different hardness from each other is frequently introduced. Recited as examples of grinding processing of composite materials are: in the field of electronics, texturing of Nixe2x80x94P plating of a hard disk substrate, uniform working of wiring metal layers and insulation films between the layers in multiple layer wiring process of LSI; and in the field of optics, grinding of the connector end faces of optical fibers consisting of a composite material composed of zirconia ceramics (so called xe2x80x9cferrulexe2x80x9d), quartz glass (so called xe2x80x9ccorexe2x80x9d of fiber) and fluoroplastics (so called xe2x80x9ccladxe2x80x9d).
With respect to a hard disk drive (i.e. a recording medium of computer), the packing density has been increased year by year. As means to attain a higher packing density, the gap or spacing between the hard disk and a magnetic head has been reduced. In other words, reduction of the spacing of the raised head has been attempted. A magnetic head mounted on a hard disk drive is generally of a thin film type magnetic head type, and examples of this type are of an inductive type, an MR-inductive complex type wherein MR (magnet resistance) is used as a recording/reproducing element, and a type using GMR (Giant MR).
These thin film type type magnetic heads are composed of a composite material comprised of a substrate such as Altic (Al2O3xe2x80x94TiC), a ceramic protective/insulation film such as alumina (Al2O3), a metallic film which is a magnetic material such as permalloy (Fexe2x80x94Ni) and Sendust (Fexe2x80x94Alxe2x80x94Si) and the like.
For example, a thin-film type magnetic head 12xe2x80x2 shown in FIGS. 1 and 2 comprises an Altic substrate 1, an alumina insulation film 2, a bottom shield film 3 (Sendust: Fexe2x80x94Alxe2x80x94Si, permalloy: Fexe2x80x94Ni or the like), an alumina film 4, an MR element 5, an alumina film 6, a head shield film 7 (permalloy or the like), an alumina film 8, a write pole tip 9 (permalloy or the like), an alumina protective film 10, and a coil conductor 11.
When a conventional free abrasive slurry is used for grinding of ABS (Air Bearing Surface) of a thin film type magnetic head, in most cases, stepped (as indicated by xe2x80x9cSTEPxe2x80x9d in FIG. 1) or rough surface results due to selective grinding or abrasion of a metallic film made of soft materials such as permalloy and Sendust, due to difference in hardness between the materials. As a result, there is a problem in that the metallic film (such as magnetic pole portion) is recessed from ABS composed of ceramics, which is called PTR (Pole Tip Recession), increasing the magnetic spacing to a recording medium (as shown in FIG. 1), thereby leading to substantial increase in raised or floating distance of the head.
In general, lubricants have been used for grinding a composite material composed of a plurality of materials having different hardness from each other, and such lubricants are classified mainly into three categories based on mechanisms of lubrication on the materials to be ground: 1) oily agent which is fatty acid such as stearic acid and oleic acid, 2) anti-abrasion agent such as phosphoric ester and Zn-DTP, and 3) organic Mo compound.
Among them, the anti-abrasion agent lowers friction even in a severe grinding condition where the oily agent loses its lubrication action. It is known that an anti-abrasion agent forms a lubricating film as the result of a tribochemical reaction with the frictional surface in a low load condition to a a high load condition at higher temperature (Seichiro Hironaka, Action mechanism of lubricant additive, Plastic Processing Symposium, 33-43,1994; R. J. Hartley et al., Anti-abrasion agent and high-pressure agent, Tribologist, 326-331, 1995).
Among such anti-abrasion agents, those compounds containing sulfur atom in its molecular chain are decomposed by frictional heat generated on the frictional surface and form an inorganic film containing sulfide between the grinding surface and the substance to be ground, thereby maintaining a lubricating property.
In the same manner, anti-abrasion agents containing a phosphorus atom in its molecular chain are decomposed by frictional heat generated on the frictional surface and form an inorganic film consisting of phosphide or phosphate between the grinding surface and the substance to be ground. It is believed that phosphite ester compound from among such compounds containing phosphorus is hydrolyzed by frictional heat to form an inorganic film. (Toshihide Omori et al., xe2x80x9cAdsorption and reaction on the iron surface of phosphorus high-pressure additives (second report)xe2x80x9d, Tribologist, 188-194, 1990).
It is known that compounds containing both sulfur and phosphorus atoms also exhibit the same effect as that of the compound comprising a sulfur atom alone or a phosphorus atom alone. (Masafumi Mashiko, xe2x80x9cChemistry and physics of lubricantxe2x80x9d, Fundamentals and applications in tribology, 15-34, 1996).
Since these inorganic films are chemisorbed to a substance to be ground, the adsorption force is stronger than that of physical adsorption of organic molecules and the film is stable up to temperatures. A large amount of energy is required in order to remove the inorganic film, and thus it is known that the lubrication effect of the film on the substance to be ground is maintained, so long as the grinding condition is not so severe that the inorganic film is removed (Toshio Sakurai, xe2x80x9cChemistry of lubrication and grindingxe2x80x9d, Lubrication, 635-642, 1982).
As an anti-abrasion agent, sulfur-containing organic molybdenum is also studied (P. C. H. Mitchell xe2x80x9cOil Soluble Moxe2x80x94S Compounds as Lubricant Additivexe2x80x9d Wear 100 (1984) 281-300; edited by Hehachiro Okabe, xe2x80x9cDevelopment and the Latest Technology of Additives for Petroleum Productsxe2x80x9d CMC (1988) p99-106; and so forth), and the possibility is studied for sulfur-containing organic molybdenum to be used as a lubricant for the grinding of a composite material composed of materials of various hardness (Japanese Patent Application No.10-255022).
As mentioned above, although the effects of anti-abrasion agents have been widely known, those anti-abrasion agents have been conventionally used for lowering the friction of the sliding surface of machine parts, such as the surface of a gear or a turbine. On the other hand, the load applied on a conventional composite material, such as a thin film type magnetic head composed of a plurality of materials of various hardness, during grinding is much lower than that on the sliding surface such as the surface of a gear and, therefore, it is quite possible to process a composite material without using an anti-abrasion agent. However, a uniform working have not been attained. Accordingly, no one has tried to add the anti-abrasion agent to a free abrasive slurry composition as an anti-selective grinding agent for the purpose of preventing selective grinding of a composite material composed of materials of various hardness as in the present invention.
Japanese Patent Application Nos. 10-113327 and 10-255022 propose the use of free abrasive slurry containing an anti-selective grinding agent which can achieve a sliding surface with high precision, thereby avoiding such selective grinding or damage.
In Japanese Patent Application No. 10-113327, prevention of selective grinding caused by lap processing of ABS of a thin film type magnetic head and improvement of the grinding surface condition are intended by adding polyether having a molecular weight of 300-20,000 and optionally polyether having hydroxyl group functionality of 1-6 obtained by addition reaction of propylene oxide with ethylene oxide, as an anti-selective grinding agent to free abrasive slurry.
Japanese Patent Application No. 10-255022 describes the use of a sulfur-containing organic molybdenum compound as an anti-selective grinding agent. The compound is decomposed by frictional heat generated during grinding processing and forms a film without causing any reaction with the metallic surface. The film has the layered structure wherein the layers are mainly composed of molybdenum disulfide (MoS2) and bonded to each other by Van der Waals force which is a weak intermolecular force. Friction at the contact area is replaced by interlayer friction in molybdenum disulfide and is lowered. As a result, the difference in the grinding amount between materials of various hardness is reduced and selective grinding of soft materials is effectively avoided.
Various free abrasive slurry compositions containing compounds having alcoholic hydroxide groups have been suggested. The conventional free abrasive slurry composition to which alcohol is added generally utilizes an aqueous dispersion. JP-A 61-261386 discloses the use of an abrasive particle-dispersion type slurry lubricant for lapping and polishing, which is obtained by mixing/dispersing abrasive particles to a medium selected from the group consisting of polyalcohol, alkyl ether of polyalcohol, allyl ether of polyalcohol and aqueous solutions thereof, for the purpose of preventing the lowering of lapping and polishing performance due to compositional change caused by the evaporation of the lubricant component during its usage. JP-A. 9-256171 discloses an aqueous oily agent composition for processing and the processing liquid containing the same, which satisfies the following properties: processability of the surface of the Ni-coated substrate, i.e. processability in plane grinding or texturing; dispersibility of the processing liquid in which abrasive particles are dispersed; non-adherence of debris to the ground film during processing or after processing (cleaning property after processing); and excellent low error property after forming a magnetic film. Further, JP-A 8-41443, 10-204419, 10-249713 or the like discloses a free abrasive slurry composition utilizing a nonionic surfactant as an grinding particle dispersant, such as polyoxyalkylene monoalkyl ether, polyoxyalkylene monoalkylphenyl ether and polyoxyalkylene monofatty ester, which is a compound having alcoholic hydroxyl groups. However, the use of these substances are not intended to prevent selective grinding of soft materials which occurs during grinding processing of the materials of various hardness and to improve grinding rate.
However, the conventional free abrasive slurry composition has preventive effects mainly on selective grinding of metallic film, and the purpose was limited to the prevention of selective grinding only, while the raised distance of the head should be further diminished and the PTR value should be further lowered. Therefore, selective grinding of the metallic film should be avoided, and at the same time, step difference (so called xe2x80x9cshoulder step differencexe2x80x9d) formed at the boundary between Altic (a substrate) and alumina (an insulation film) during grinding processing using free abrasive slurry should be lessened. In addition, in view of grinding ability of free abrasive slurry, there is also a problem in that, when lubricating property is enhanced too much, grinding property may be lowered and thus the grinding rate of free abrasive slurry may be lowered.
The object of the present invention is to provide a free abrasive slurry composition which is advantageously used during grinding process in which a composite material comprised of materials of various hardness is uniformly ground without causing difference in the grinding amount between materials of different hardness, i.e. selective grinding.
Another object of the present invention is to provide a free abrasive slurry composition which is advantageously used in uniform grinding of a thin film type magnetic head without causing scratch on ABS of the thin film type magnetic head; and the method of grinding using the same.
The present invention provides a free abrasive slurry composition which is advantageously used during processing step (especially of a thin film type magnetic head) in which a composite material comprising materials of various hardness is uniformly ground without causing difference in the grinding amount between materials of different hardness, i.e. selective grinding, wherein the high grinding rate is maintained and the high grinding quality is attained; and an grinding method using the same.
A subject of the present invention is to provide a free abrasive slurry composition for grinding a composite material comprising materials of various hardness, said composition containing abrasive particles, an anti-abrasion agent as an anti-selective grinding agent containing at least one of sulfur and a phosphorus in its molecular chain (except sulfur-containing organic molybdenum) and a dispersion medium. Preferably the anti-selective grinding agent is added in an amount of 0.1% by weight or more in the slurry composition. Preferably, the anti-selective grinding agent may further contain at least one of members selected from the group consisting of a surfactant, a macromolecular surface modifier and a coupling agent,
Another subject of the present invention is to provide a method for producing a thin film type type magnetic head including the step of grinding the surface which is to be an ABS (air bearing surface), wherein one of the above-mentioned free abrasive slurry compositions is used in the grinding process.
By using such free abrasive slurry composition, the area of solid-solid contact is reduced, thereby selectively lowering friction coefficient at the surface of the substance of lower hardness to be ground, during the grinding step of a composite material composed of a plurality of materials of various hardness. In other words, by reducing the removal amount of the materials of lower hardness, uniform grinding of the material becomes possible without causing difference in the grinding amount between material of various hardness.
Another subject of the present invention is to provide a free abrasive slurry composition for grinding a composite material composed of materials of various hardness, which comprises abrasive particles, a dispersion medium and at least one compound having alcoholic hydroxyl group(s), and optionally, a dispersant.
The present invention is a free abrasive slurry composition wherein the above-mentioned compound having alcoholic hydroxyl group(s) is primaryxe2x80x94tertiary alcohol represented by the following formula (I):
R1xe2x80x94OHxe2x80x83xe2x80x83(I)
(wherein R1 represents a C4-C18 alkyl group, an alkenyl group, an alkyl aryl group or an aryl group); or
a free abrasive slurry composition wherein the above-mentioned compound having alcoholic hydroxyl group(s) is polyoxyalkylene monoether compound represented by the following formula (II):
R2Oxe2x80x94(CnH2n+1O)mHxe2x80x83xe2x80x83(II)
where R2 represent a C1-C18 alkyl group, an alkenyl group, an alkynyl group, an alkyl aryl group or an aryl group; n is an integer from 2 to 4; and m is an integer from 1 to 4); or
a free abrasive slurry composition wherein the above-mentioned compound having alcoholic hydroxyl group(s) is a polyoxyalkylene monoester compound represented by the following formula (III):
R3COOxe2x80x94(CnH2n+1O)mHxe2x80x83xe2x80x83(III)
(wherein R3 represents a C1-C18 alkyl group; n is an integer from 2 to 4; and m is an integer from 1 to 4).
Another aspect of the present invention is a method for producing a thin film type magnetic head including the step of grinding processing of the surface which is to be an ABS of the thin film type magnetic head, wherein one of the above-mentioned free abrasive slurry composition is used in the grinding processing.
The feature of the present invention is to avoid selective grinding of a metallic film and step difference between Altic and alumina, by using the above-mentioned free abrasive slurry composition.
When a compound having alcoholic hydroxyl group(s) is used as an anti-selective grinding agent in the present invention, it is considered that selective grinding is reduced since an alcoholic hydroxyl group, which is a polar group in the molecule, is selectively adsorbed to the metallic surface. On the other hand, with respect to Altic (a substrate) and alumina (an insulation and protective film), both interact less with the compound having alcoholic hydroxyl group(s) as compared with the metal, there is no notable difference in adsorption of the compound having alcoholic hydroxyl group(s). Therefore, it is believed that the compound has an excellent capability in avoiding selective grinding between Altic and alumina. In addition, adsorption of the alcoholic hydroxyl group to the solid surface is weaker as compared with adsorption of a carboxyl group, and thus it is expected that grinding processing in which higher grinding rate is maintained is possible by using the compound.
Accordingly, it is effective to add at least one compound having alcoholic hydroxyl group(s) to a free abrasive slurry composition in order to attain uniform grinding without causing difference in the grinding amount between materials of various hardness during grinding a composite material, and to maintain effectiveness of grinding of free abrasive slurry, i.e. higher grinding rate.